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David originally posted this as a Google Doc, and I’m reproducing his work here with his permission.

Just the other day I was speaking to a climate change skeptic who made mention of an old Time or Newsweek (he was not sure) article that talked about fears of a coming ice age.

Derivative cells that mature toward the inside of the cambium initial cell become secondary xylem.

Derivative cells that mature toward the outside of the cambium initial cell become secondary phloem.

A few days ago a facebook friend of mine posted the following image: From the 1977 cover we can see that apparently a new ice age was supposed to arrive.

Only 30 years later, according to the 2006 cover, global warming is supposed to be the problem. It actually is this Time cover from April 9, 2007: As you can see, the cover title has nothing to do with an imminent ice age, it’s about global warming, as we might expect from a 2007 Time magazine.

In looking at a cross section of a one- to three-year old stem, the secondary xylem and secondary phloem cells are easy to distinguish from their primary xylem and primary phloem counterparts.

And so while we are busy patting ourselves on the back or passing laws to prevent cloning, plants have been doing it naturally for millenia.

This forms a continuous cylinder of cambium in the stem.

You might wonder how the cambium could expand across the pith ray which is already filled with mature parenchyma cells.

The hormonal/developmental pathway is not completely clear yet, but those mature parenchyma cells dedifferentiate (go "backwards" in development) into meristematic cells.

The ability of mature parenchyma cells to do this is truly remarkable.